1. Field of the Invention
The present invention relates to a bonding apparatus such as a wire bonding apparatus, die bonding apparatus, etc.
2. Prior Art
Conventionally, bonding apparatuses in which a vibration is applied to the bonding tool during bonding have been classified into three main types as described below.
In the first type, an XY table installed in the bonding apparatus is vibrated so that a scrubbing action is performed. This type of bonding apparatus is disclosed in Japanese Patent Nos. 2530224 and 2565009.
In the second type, a scrubbing action and an ultrasonic vibration for bonding are combined using a single piezo-electric element. This type of bonding apparatus is disclosed in Japanese Patent Application Laid-Open (Kokai) Nos. S62-249437 and S63-239834.
In the third type of apparatus, a plurality of ultrasonic oscillating elements are provided, and bonding is performed by applying ultrasound using a combination of vibration of the oscillation elements. For example, a bonding apparatus of this type is disclosed in Japanese Patent Application Laid-Open (Kokai) No. S58-161334.
In the case of the scrubbing action performed by means of an XY table in the above-described first type of apparatus, the large-mass XY table and the bonding head as a whole are moved. Accordingly, vibration of the apparatus as a whole causes a fluctuation in the bonding load, and precise control of the amplitude and frequency of the scrubbing action is difficult. These and other factors hinder a stable bonding operation.
In systems of the second type described above in which ultrasonic vibration and a scrubbing action are combined by the use of a single piezo-electric element, there is a large difference between the ultrasonic oscillation and the scrubbing action in terms of the amplitude required. Accordingly, if a piezo-electric element which is suitable for the ultrasonic oscillation is used, the amount of displacement is too small for the scrubbing action. In the case of a multiple laminated type piezo-electric element in which the amount of displacement is large, the internal loss is large, and a large loss results when such an element is used for ultrasonic oscillation. Furthermore, as the number of laminated layers increases, i. e., as the amount of displacement is increased, the resonance frequency of the piezo-electric element decreases, so that the piezo-electric element becomes unsuitable for ultrasonic oscillation.
In systems of the above described third type which have a plurality of ultrasonic oscillating elements, a scrubbing action is not performed, and the following problems arise:
In particular, the amplitude of the ultrasonic oscillation gradually increases from an extremely small value, and the increase in amplitude per cycle of vibration is extremely small. For example, assuming that the ultrasonic waves rise up to 1 micron in 0.5 ms; then if the frequency is 60 Khz, the increase in amplitude is 1/30 micron per stroke, because there are approximately 30 amplitude before rising up to one micron. When the wire is pressed against the bonding surface by the bonding tool, the surface of the wire that contacts the bending surface bites into projections, forming indentations on the bonding surface. When ultrasound is applied in this state, the portion of the surface of the wire where the projections and indentations are bitten into at the first amplitude (peak) is plastically deformed only by an amount that corresponds to a small amplitude. Since the amplitude gradually increases from this state in each cycle; the deformation of the wire surface spreads by an extremely small increment in amplitude in each cycle.
Thus, even if the amplitude reaches a maximum while the deformation gradually spreads, the amount of movement by which the surface of the wire attempts to destroy the oxide film on the bonding surface is only this extremely small increment of one cycle; accordingly, the oxide film on the bonding surface merely undergoes elastic deformation, and this action is insufficient to destroy the oxide film.
In the case of scrubbing, on the other hand, the stroke of the first amplitude (peak) is much larger than in the case of ultrasonic oscillation and is therefore sufficient to destroy the oxide film so that bonding is made possible. Such superiority of the scrubbing action is due not only to the fact that the amplitude is several times that of scrubbing effected by ultrasonic oscillation, but also due to the fact that the initial distance of movement of the amplitude is completely different.